text:
Helicopters: why hasn't NOTAR been more popular?
NOTAR
No tail rotor: use of a ducted fan within the helicopter's body, and venting the air through the side of the tail boom to get the desired anti-torque.
One big advantage is simplicity and ruggedness: no tail rotor to get damaged, no tail rotor gearbox or driveshaft to wear and/or malfunction.
Another is noise: NOTAR is quieter.
And maybe lower cost: the simpler design and fewer components moving under high torque should be less expensive to build, definitely less expensive to maintain.
One possible disadvantage might be fuel consumption - it is quite possible that the ducted fan and the air turning a corner isn't as efficient as a propeller, so fuel consumption might be higher, but not that much higher. I haven't seen any figures on that, one way or another.
As of today, only Boeing (via purchase of M-D) uses the ducted fan in three MD helicopters, and even they haven't sold that many helicopters with this design. In all my aircraft watching, I have seen only one MD 520N in flight.
I'm just curious - other than maybe patent royalties, is there a mechanical reason this design hasn't been more popular? On the safety and maintenance aspects alone, it seems like a winner. And for military helicopters, the lower vulnerability and lower radar return would seem like big advantages.

Though NOTAR looks simple from outside, its actually a bit complicated. Let me use the following figure to elaborate on this point.

By Voytek S - Own workThis vector image was created with Inkscape., CC BY-SA 3.0, Link
Details from the figure: 1. Air intake
    2. Variable pitch fan
    3. Tail boom with Coanda Slots
    4. Vertical stabilizers
    5. Direct jet thruster
    6. Downwash
    7. Circulation control tailboom cross-section
    8. Anti-torque lift
You have the ducted fan (2), which is driven using power from the gearbox, which is used to produce the anti-torque effect through Coandă effect. Note that this fan has a variable pitch, which is adjusted similar to the tail rotor.
The Coandă effect actually provides only about two thirds of the required anti torque. The other one third is provided by the directed jet thruster (5) which rotates about the tail boom axis (that is why it is projecting outside) like a rotating can.
The problem is that in cruise, the effect is markedly reduced by the airflow over the tail boom, which is overcome by making the vertical stabilizers (4) adjustable. So basically, you have three systems for adjusting the anti-torque, compared to one in the conventional tail rotor. More systems translates into more parts, more complexity, more chances of failure and more maintenance, not to mention that this makes the helicopter tail heavy. For example, when using the same engine, the NOTAR MD 520N is heavier than the conventional MD 500E (from which it was developed) (Source:1, 2).
As you've noted, this system is not as fuel efficient and sacrifices speed and range for lesser noise and improved safety. Its a tradeoff and most operator would take the better performance over others.
In case of military helicopters, the main problem for helicopters, the infra-red detection, is improved in case of NOTAR only if the engine exhaust (or part of it) is routed through the fan, as the South Africans did with the Aérospatiale Alouette III III in their Cirstel (Combined Infra Red Suppression and Tail rotor Elimination) project.

Denel Cristel; image from stingrayslistofrotorcraft
As the site explains,

The Cirstel principle splits the high pressure air from the engine for use in the Coanda slots, while the low pressure air is bled-off for the tail thruster and to mix and cool the engine exhaust. The thruster nozzle is a Denel-patented clamshell design, unlike McDonnell Douglas' `rotating can' concept.

As you can see, the system is further complicated. In addition, routing the hot air through the tail boom brings its own headaches and is better avoided. So, even in that sense, the advantage is not striking.

Ahh, beat me to it.  Nice!  I remember there being some decrease in control authority with an increase in forward flight speed as well (since the rotor downwash is deflected further astern), though I'm having issues finding citations for that right now.
Would you have any info on that as well?

Ah, so it wasn't so simple, after all. Nothing involving rotorcraft ever is.

+1 for that image alone. That is one of the best explanatory images I've ever seen.

That graphic you drew is very impressive.

I was an aerospace engineering student in the early 80's in the Phoenix, Arizona area and was privileged to see some of the early development done by Hughes Helicopters.  For me, there was nothing more impressive than seeing a pilot back the tail rotor of this machine into a hedgerow of trees until it was buried in the vegetation.  It still boggles the mind to think about it.

The fan-in-fin has been very popular with Eurocopter, most of their models have this arrangement, which they call fenestron. This one is from the EC-135, photo from the wiki page. However, they mount the fan sideways in the open, not hidden in the tail boom. One could argue that the fan-in-fin is functionally identical to a tail rotor - it has some of the advantages that you mention like lower noise, less susceptibility to blade strike, flow not impeded by vertical tail, and tail flow shielded from forward airspeed.

The MD 520 is a true NOTAR, it does not have an external fan but a kind of side jet out of the tail boom. Early designs used a straight jet which was kind of expensive, the Hughes design uses the Coanda effect to deflect the main rotor downwash sideways, thus providing a side force in the hover. At higher forward velocities this becomes less effective and the vertical fin needs to take over directional stability, together with a jet thruster.

If the turbine exhaust can be used directly for the NOTAR arrangement fuel economy would definitely be better than tail rotor, if the NOTAR fan is driven by the gear box there is no clear advantage over fan-in-fin other than the sound issue:

One big advantage is simplicity and ruggedness: no tail rotor to get damaged, no tail rotor gearbox or driveshaft to wear and/or malfunction. It's not really that simple, with gear-box driven fan with variable pitch, vertical tail with variable incidence, and jet blaster with variable output.

Another is noise: NOTAR is quieter. True. But how important is it?

And maybe lower cost: the simpler design and fewer components moving under high torque should be less expensive to build, definitely less expensive to maintain. It's not really that simple...

Your question on why true NOTAR is not used more frequently probably boils down to proven concepts being more trusted than innovative concepts in the field of aeronautics, if there is no very clear advantage. One could ask: why mount the fan internally, why not in the fin?
Image from Principles of Helicopter Aerodynamics by J. Gordon Leishman.

Do all helicopters benefit from the Coanda effect? I notice that tail-rotor choppers seem to have a less-cylindrical tail boom, whereas the NOTAR ones have a more cigar-shaped boom - obviously to maximize the effect, but is there a disadvantage to optimizing for the Coanada effect when you already have a tail-rotor?

@Dai you'd need an internal fan in addition to the tail rotor to benefit from the Coandă effect. I.e., worst of both worlds.


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Why is a (jet) engine in flight shutdown so critical?
EASA just announced an Emergency Airworthiness Directive grounding all A320neo planes with a certain type of engine. Reason is, that they have apparently the tendency to shut down in flight more often than acceptable.
I was wondering, why are IFSDs so critical? Can't a jet engine be just restarted? If not, is it something with the inherent design of jet engines? Or something that most engines are just designed for?

If you read your question again, you see the cause of such action is not "IFSD" but "IFSD more often than acceptable". So if you question the necessity of this action, you are effectively arguing the definition of "acceptable". When that's the case, you need to provide the current definition and your exception to make this question answerable.

IMO unexpected IFSD can have pretty severe consequences so I guess the tolerance of its occurrence should be relatively low, lower than say a phone or computer shut down unexpectedly. In this case the occurrence is "several occurrence" out of less than 100 airplanes of this model, I would say that's dangerously high for almost any product.

It's not enough to make an answer of, but I'll point out that having an IFSD may not be a big deal in certain cases, but to take off knowing that an IFSD is more likely in your particular model of aircraft is crazy.

This is a comment as it probably doesn't apply to engines now, but prior to my retirement in 1999, if at altitude and you pulled the power too fast on Pratt and Whitney JT9D engines of the time, they would occasionally quit. The procedure was to tell the flight engineer to restart it once the aircraft was low enough that that was possible.

@nexus_2006 Not really. Risk is comprehensively modeled and quantified in aviation, and most safety-critical engineering. There's a real and known risk that the airplane will crater killing everybody aboard, it's just been managed to an acceptable level (<1E-9/flight hour). On a single-engine aircraft, that number is <1E-6/FH, they are known to be 1000x more dangerous, but we take that as acceptable. A limit is set, and when things exceed the limit, as in this case with dual engine failure, people take action.

With an increased chance of engine shutdowns, ETOPS is the primary concern. Grounding all of them to investigate is a logical outcome.

Becaaaaaause... the engines are the only thing holding the plane up in the air?

@DavidRicherby Actually... the aerodynamic surfaces are the only thing holding the plane in the air. The engines help the aerodynamic surfaces do their job. ;-)

Having an engine shutdown during the flight is an engine failure. An engine failure is one of the procedures where every pilot is trained to deal with because it is easy to simulate. Engine failure is an emergency, meaning that the pilots must land the aircraft as soon as possible.

Can't a jet engine be just restarted?

Maybe, maybe not. It depends on the diagnostics in the cockpit. You certainly do not want to restart a damaged engine. All jet engines are capable of being shut down and restarted in flight, provided that nothing is broken and proper procedures are followed.
Now, of course, if everything was working then the engine would not have shut down in the first place. It is an event which pilots are prepared for, but it should not have occurred. When it does, that means something is wrong. The decision of whether to attempt a restart is evaluated by the flight crew based on engine data, distance to nearest airport, weather, and possibly a discussion with ground support. In short, it is a risk management exercise.

Why are in flight shutdowns so critical?

With one engine shut down, your chances of making it to the destination are virtually none. Aircraft performance is calculated assuming that all engines are working. With one engine shut down, the amount of thrust is reduced and the aircraft must descend to a lower altitude. At lower altitude, fuel consumption is higher, therefore you will not have enough fuel to reach your destination and you need to land somewhere closer.
Furthermore, you are now running on your last good engine. If that engine fails as well, the airplane will become a glider. ETOPS is designed so that an aircraft may limp to the nearest airport in no more than X minutes, and the presumption of ETOPS is that the engine failure rate is low enough. In other words, the aircraft failed to meet its certification standards.

This particular incident has RTOs involved so probably the answer should add that, too.

While an engine failure on a two-engine aircraft calls for a landing as soon as safely possible, the failure of an engine on a four-engine aircraft, at least 747s up to my retirement in 1999, did not require a landing. Indeed, 3-engine ferry flights are permissible, where you take off with an engine out. They are practiced in the sim, and I did three as I remember for real.

@Terry You can continue OEI on a triple too, and they are two-engine ferried as well. 14 CFR 121.565 refers in the US.

@user71659 I figured that was the case, but didn't want to mention it as I wasn't certain, and especially with dementia increasing the rate of my 'senior moments'. LOL

Also when the cause is unknown, it cannot be excluded both engines will experience the same failure at the same time . E.g. the cause is an erratic failure of a sensor in particular atmosphere conditions, both engines can experiment the sensor failure at the same time.

As pointed out already, this answer (particularly the second half of it) may well be applicable to two-engine aircraft such as the A320neo, but for three- or four-engine aircraft the loss of a single engine wouldn't necessarily be immediately critical. Particularly, if the engine shutdown happens late in flight, the destination airport might be the most suitable airport to aim for, while letting ATC know that you can't tolerate any delay because you're going to, at least, be tapping into your reserve fuel. You may want to emphasize that.

@Terry yes, there was an incident where a BA 747 had an engine failure on takeoff roll at LAX, and decided to divert... to Manchester.

@Kevin is quite correct but to add to it the airworthiness issuance was issued because of the risk of a dual engine failure

...issued an emergency airworthiness directive (AD) warning of a
  potential “dual engine” inflight shutdown on A320neo family aircraft
  powered by Pratt & Whitney PW1100G geared turbofan (GTF) engines.

@Kevin covers the single engine failure shutdown well, but in this case a dual engine failure could potentially be catastrophic if there was no useable landing site within gliding distance. Although evidence has shown that dual engine failures can end safely like this one and this one

and this one: https://en.wikipedia.org/wiki/Air_Transat_Flight_236

Honestly not sure that I'd call either of those incidents "safe." "Survivable," yes, but an accident in which the airframe is heavily damaged or destroyed and a few people receive serious injuries doesn't really fall within my personal definition of "safe." :) And it was really only due to excellent airmanship (and some luck with regard to location and altitude at the time of the failures) that they were survivable.

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How late can a landing clearance be given?
I regularly fly VFR in an (EASA-) airport where the tower is sometimes rather busy and the frequency full which led a few times to me receiving landing clearances as late as 10-20ft AGL above the threshold.
I was wondering, if (EASA) regulations say anything about when you have to / should perform a go around on approach when you still didn’t get a clearance. I’m not asking about how to deal with those situations when flying, I’m just asking about what’s in the regulations about this.

Hopefully before you touch down!

The approach clearance (be it VFR or IFR) has a clearance limit until over the threshold. So, you cannot cross the landing runway threshold unless you have received landing clearance.
Approach clearance always includes clearance to fly the missed approach procedure.

Since Florian is asking specifically about EASA regulations, it would be a good idea if you could cite the relevant regulation(s) in your answer.

Not true. You can't LAND without a clearance to do so, but clearance for an approach is also clearance to fly the missed approach if you can't land (for whatever reason), so you absolutely DO have clearance to overfly the threshold, having been cleared for the approach.

@RalphJ You are absolutely correct. I left it out on purpose as the question was only about landing, but I'll edit my answer to include this.

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Why don't aircraft fly direct via oceanic airspace from UK to Iceland?
Whenever I watch the routes of aircraft flying between BIKF (Keflavik, Iceland) and e.g. EGLL (Heathrow, UK), they make a small turn in between to avoid oceanic airspace. See for example this flight route:

(Source: Screenshot of https://www.flightradar24.com/data/flights/ba894#236858ea)
The turn is made at the point where Shanwick Oceanic, Reykjavik FIR and UK airspace border each other. A direct routing would have a segment of around 220NM in Shanwick Oceanic airspace.
What's the reason, these flights don't go through Oceanic airspace? I observed that it's operator independent (ICE, BAW, EZY all fly this dented route) and it doesn't correlate with aircraft capabilities (ICE flies the same airctaft through Gander Oceanic on the way to North America all the time). So why is this routing?

Oceanic, you say? Remember what happened to Oceanic 815...

When in doubt: fly over land.

@Mast "Oceanic" here means who controls the airspace, not literally flying over land vs. over water. They're flying over water either way.

Interestingly, based on the answer below it seems it can be summed up as "Because stupid." ie in this modern high tech age we are artiificially routing to meet some semi-random paper requirement. || In 2003 I flew London-Bangkok on QANTAS. Rather than taking the obvious route across Iran, we flew to the north of Iran parallel to the border and then turned slightly 'right' thus avoiding entering Iranian airspace. I imagine that that would be still be being done at this immediate moment :-(.

I don't think they are specifically avoiding Shanwick Oceanic airspace. The reason seems to be related to the airways in the Scottish airspace. In general, you have to file a route using airways when operating in this airspace and there simply isn't one available that provides a more direct route. I recreated what looks like the route you show on skyvector.com:

This route would use UL612 between DCS (Dean Cross) and STN (Stornoway) and then continues via the N610 to RATSU, which marks the border between Scottish, Shanwick Oceanic and Reykjavik airspace. There are no airways from RATSU in the Reykjavik airspace, so they probably filed a direct route from there. The total length of this route would 1049 NM.
You can see alternative airways to use in this zoom (or go to skyvector.com, select "World Hi" for high altitude airways and zoom in):

There are two waypoints South of RATSU where one could enter Shanwick Oceanic airspace: ATSIX and BALIX. The shortest routes I could find with skyvector were 1042 NM and 1035 NM long respectively. This is basically the same distance and the optimal route would likely depend on the current winds.

I feel like the important part of your answer is “you have to file a flight plan using airways when operating in this airspace” which I wasn’t aware of, I thought it’s possible to use DCT routings between BIKF and EGLL (or the respective STAR and SID fixes)

It is sometimes possible to file DCT segments, but this might get rejected when validating the flight plan with Eurocontrol.

@Florian Generally speaking, you must always file along airways, unless the airspace in question has implemented Free Route Airspace

Fair enough, but this invites the follow-on question: why isn't there a more direct airway?  Surely the waypoints exist to serve flight needs, not vice versa.  Or perhaps this route actually is quite direct on a great circle? and only appears especially dog-legged on this projection?

There's probably a lot more transatlantic traffic than traffic to/from Iceland, so the routes in this area are probably optimised for the former, with the added complexity that the transatlantic routes change depending on jet-stream. Also, isn't there a need to pay a fee for each ATC area one flies through?

@SwissFrank The projection won't add a dog leg, but the great arc path isn't too far off of this path. The great arc path from LHR to KEF is 1,025 nmi, only 24 nmi shorter than this path. They probably simply figured that it's not worth adding complexity to the oceanic airspace system and reducing flexibility for the North Atlantic Tracks just to save 20-25 nmi on flights to Iceland.

Thx Reirab but 2% savings in fuel (or given that climb uses a lot of fuel, maybe 1%?) still isn't to be ignored, if the cost of gaining this advantage is merely adding or removing a waypoint on some official list...

The airspace/airway layout could have to do with oceanic entry and exit points being spaced to accommodate oceanic track separation requirements. Having airways cut across the entry points to the North Atlantic Tracks (which themselves are redefined on a daily basis to match the current weather) might be considered as adding too much complexity. Just speculating, of course.

I just dived deep into it and the most efficient route given other things like routing inside FIR Reykjavík would be to fly inside UK airspace BALIX EGNO EGLL (which is not along airways), but that would only be a 1% improvement to flying via BALIX and then UK airways. So For UK the 1% is probably fine. For airlines, flying via RATSU gives only 1% more way length than via BALIX and has the benefit of just flying through 3 FIRs (Reykjavík, Scotland, London) vs. 4 (Reykjavík, Shanwick, Scotland, London), so less complexity. So no side has a high enough incentive to change something unilaterally.


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What is the input range of common fly-by-wire controls?
What's the range that typical fly-by-wire controls (let's take A320 and B777 as examples) allow inputs in?
The A320 controls in the y-axis of the side stick (forward/backward axis) the g load - what's the minimal g load you can input when pushing the stick fully forward / the maximum when pulling it fully backwards? Is this a fixed value or does it depend on factors like speed (IAS / TAS / ...), attitude, altitude / air density or anything / ...? Or is it just the maximum value that the flight controls could give you at any moment?
Same for roll rate, what's the maximum you can command there? And for the B777, it's basically the same question except for the yoke instead of the sidestick.
Also, is the relation between control input (side stick movement / yoke movement) and controlled variable (g-force, roll rate, ...) a linear curve? I.e., if the side stick gives you at the neutral position 1g and e.g. 1cm backwards it gives you 1.1g, does 2cm backwards give you 1.2g?

Fly by wire systems do not define maximums by the deflection of the control input. The system will detect what the pilot wants to do but in extreme cases will limit the changes and rate of those changes in attitude to the safe limits of the aircraft.


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What are those sensors on the forward A321neo fuselage?

What are those sensors on the forward (Airbus) A321neo fuselage? If it helps, this is WOW air's TF-DTR.

And the yellow antenna behind the Ice Detector is the ATC (Air Traffic Control) antenna.


text:
Are there (ideas for) GNSS based attitude indicators?
With upcoming GNSS systems that have better accuracy than GPS, could there be an GNSS based attitude indicator? Does maybe one already exist?
I could imagine having 4 sensors (front, aft, left wingtip, right wingtip) that then calculate the attitude.

Then what would be your backup in case of a GNSS/GPS outage?    Part of the reason for Gyros is to have separate systems that can be cross-checked against each other.

Even today, differential GPS accuracy is about 10cm, which is perfectly sufficient. The inaccuracy from wing bending is larger than that.

Researchers at Stanford University had working prototypes 20 years ago. They worked, but not very practical or cost effective when compared to a solid state ADHRS which came about the same time.

I visited a search and rescue cruiser some years ago that used three GPS receivers to calculate true heading at sea.

Which GPS signal in particular are you referring too? All of them in general?

Part of the attitude determination on the ISS is done that way.  The size of the space station makes for a larger measurement base then on an aircraft, but the idea is the same.  Further, the ISS has an unobstructed view of the navigation satellites, and has lower rotation rates than one can expect on an aircraft, which makes it an easier problem.  But, a quick search finds plenty of research for doing the same for aircraft.

I suppose you could but what benefit wound a GNSS based system offer over traditional gyroscopic and AHARS based attitudes systems while offering better redundancy against a system reliant on outside radio sources and risks with system outages, etc?

There has been quite a bit of research on the topic and units have even been built. Ultimately it will trickle its way out there if the price of the unit drops below AHARS or Gyro equivalent units and its accuracy is equal or better than modern AI's. However you can get a good, small, drop in, AHARS style AI for fairly reasonable (considering the cost of avionics). The FAA seems to allow GNSS signals to supplement AHARS data I cant find a copy of TSO-C4c which outlines the certification limits for pitch and bank instruments but may shed more light on the topic.
Side note: to escape the mechanical dependence of gyros and if you want something different than what your run of the mill AHARS unit has, some of the newer high performance stuff uses a Ring Laser Gyro.

This isn't a direct answer to your question, but there has been research on a simpler gyro-free and GPS-free absolute attitude determination method based on a trio of 3-axis magnetometers and a trio of 3-axis accelerometers.
Very much like you imagine 4 GNSS sensors, they use a trio of 3-axis magnetometers -- each detecting the 3-axis direction of magnetic north.
They don't need to be at the extremities of the aircraft, just separated in space for sufficient magnetic and rotational accuracy.
The challenge to overcome lies with the fact that "the aircraft environment is very poor for the performance of the magnetometer, with large electromagnetic transients that cannot be calibrated out of the measurements". Their solution is to integrate a trio of accelerometers, and a time-filtered sampling method to reject transient magnetic readings.
They validated with limited test flights, but there is insufficient information to know how it might hold up to less-than-transient magnetic phenomena in the real world due to storms, static charges, etc.
The paper was published by Stanford in 2000...
A gyro-free quaternion-based attitude determination system suitable for implementation using low cost sensors
One future possibility might be to create an attitude detection system which incorporated all three methods (relative "digital" laser-gyro, motion extraction from a trio of accelerometers, vector extraction from a trio of 3-axis magnetometers, and absolute position extraction from four extremity mounted WAAS GNSS sensors), and used each to backup and reject spurious data from the others.
This would not only provide the least likelyhood of inaccurate attitude indication, but it would also provide extensive data on when there is divergence between the different methods.


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Why are emergency-instructions by cabin crew like "heads down!" not given via PA?
I just saw a YouTube video of a landing airliner with the gear not fully extended and the cabin crew gave the instructions "Stay down! Heads down!" repeatedly to passengers but without any technical help like PA. Why is that the case? Why don't they use the PA system for commands like that?

I'm going to guess that the PA is left unused because there might be a need for other announcements that override the "stay down" yells. Particularly the pilots might need to say something, like "prepare for water landing" or "evacuate now". The pilots don't have any other way of communicating.

Not knowing that specific video, impossible to tell the scenario.
But in general, you want to not have to rely on technology in an emergency situation. What if you were trained to use the PA system but there is no electricity?
So you train to shout and use non-verbal communications to get your point across in an emergency.

She is strapped in and won't be holding the PA handset in the final moments because it can be a dangerous missile in a crash.
And they can get by without it.  Most of an FA's training is in how to speak, project and dominate in chaotic situations.  They are effectively riot cops you might say, with extensive training on crowd/person control, who are likely to spend their entire careers working undercover as waiters/stewards until that one in a million moment comes along like in that video.

Nonsense... they absolutely have access to the PA microphone while seated & strapped in to the jumpseats.  Lack of access to the mic is most certainly NOT a correct answer here.

Yes my mistake.  By lack of access I meant it's more of an SOP thing where they are not supposed to hold the handset when about to crash because it's going to be a fairly heavy flying object.

Your hands also become a dangerous missile in a crash. Why there's a specific hand position when bracing, which is not holding a handset to your face.


text:
Do all A320-family planes have leading edge high lift devices?
I keep seeing pictures of A320/321 planes with slats or some other form of leading edge high lift devices. However, on all my flights in the last years with A320s/321s I never noticed anything changing on the leading edge during approach (from my pax window point of view).
So my question is: Is that an option airlines can choose, if they want to have leading edge high lift devices? Or are they in all aircraft of the family? If so, are operators flexible in using them for approaches?

Most Airlines have slats, as a matter of fact, even smaller jets have them. Slats have to do with swept wings which  allow to fly faster.
Manufacturers use this method to delay the onset of relative wind on the upper side becoming supersonic. The trade off for this is (amongst other things)a higher stall speed and a higher angle of attack required at approach speed or close. Slats together with flaps help circumvent this problem allowing the aircraft to fly slower.
On the Falcon 2000 (my current A/C) the slats and flaps come together and we can’t control one or the other ( unless in an emergency) it’s both at the same time i.e slats extend when the first stage of flaps is selected i believe it’s the same on all airliners including the A320.


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Why does the author not add the pressure of the freestream to the variables (dimensional analysis)?
This page (page 35) is from "Fundamentals of aerodynamics" by John D. Anderson, Jr (Fifth edition):

Why does the author not add pressure of the freestream (static pressure for clarify) to the variables ? How does he know that pressure of the freestream not effect the aerodynamic force R?

stop posting question that are only images. type the relevant parts here. equations can be typed with mathjax

Thank you for your concern. But I think this kind of question is better when I just post the image of the text, it assure the original of the text. I mean: no need to deal with formulars and special texts with this kind of theoretical question. Please give me more comment if I am wrong about this

no, it is not better and we said it multiple times. Images of text cannot be searched in the future if we ever want to reference this question to other users, making it less useful.

Oh I understood: the question needs to be readable by the seach machine to help other people to find the same question, right?? I am so sorry for this, I swear I will change in next question

Static pressure is everywhere. At the front of the wing where the air streams onto, but also at the back of the wing.
We're talking on an intuitive basis here, and we do realise that the denser the fluid, the higher the force. Lift will be higher in water than in air, it is 1000 x denser. But it does not increase if we go from 10 m below sea level (pressure = 2 bar) to 20 m deep (pressure = 3 bar).
Static pressure will become a factor when compressibility needs to ne considered, near transsonic speeds and above. How much the air is compressed does depend on static pressure. The analysis with density and velocity only holds for lower free stream speeds where the air can be considered incompressible. Indeed, point 5 takes this into account.
Update
Friction drag of a fluid is caused by the viscosity of the fluid. According to Viscopedia, static pressure has a very small influence on the viscosity of fluids that may be considered as incompressible. Air at lower Mach number is usually considered incompressible.

Here we have aerodynamic force R is lift and drag. Maybe the lift is not increased when you go under below, but due to pressure increasing, the drag may be different because the fluid molecules press harder to the surface of the body.
And not like you say in second paragraph,the compressiblility is already considered by the 5th variable: the freestream speed of sound, not the static pressure

molecules pressing harder on the surface would be skin drag, not induced drag, and therefore not part of R

@DatXaLin ..but due to pressure increasing,..That's dynamic pressure. Static pressure is everywhere, also on the back of the wing, pressing forward. It's a random movement of molecules, in all directions. Dynamic pressure is a velocity component superimposed on the random movement. Total pressure is the sum of dynamic pressure and static pressure and is measured on the leading edge of the wing. And yes, variable 5 takes compressibility into account so for the dimensional analysis that is enough.

@Radu094 that's not a good idea, skin drag still be the part of aerodynamic force R

@Koyovis  "but due to pressure increasing", I meant that's stactic pressure, not dynamic pressure. When you go below the sea level, the static pressure increase, right ?? And like I comment before, this pressure will press harder to the surface of the body, then the drag force maybe change. That's why I think the static pressure of the fluid need to take into account.

But the static pressure presses everywhere on the body, in all directions. Its net effect is zero force.

I know the static pressure presses everywhere, but it presses with a moving fluid, there  can be a chane it will effect the skin drag or other force. Analogously, imagine you have a piece of steel moving in a table, if you press the piece of steel, the friction force will increase

Friction drag is caused by viscosity. Please refer to updated answer.

Freestream static pressure is not an independent variable when both density and speed of sound are given already. Adding pressure will not improve the equation because it can be canceled out at a later stage.
Static pressure is linked to density and temperature via the ideal gas law. Temperature can be deduced from the speed of sound.

that's good point, I also noticed that but there still is a little confuse. That's true for gases. In the liquid, you can easily change the static pressure (adding more pressure on the surface of the liquid) and keep the density and temperature stay the same. So pressure in the liquid is independent, right ??

@DatXaLin: Liquids are less compressible than gasses, but not completely incompressible. But you are right, my answer was written with only gasses at low or moderate pressure in mind.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
People in a rollercoaster or falling elevator feel the rapid descent, so why don't pilots in the cockpit?
Judging from the CVR transcript, the AF447 pilots didn't realize for most of the plunge that they were stalled and rapidly losing altitude.
Why don't pilots in the cockpit get the same sensation of falling down as people in a rollercoaster or a falling elevator ? Both are situations in which you would unmistakeably know you are falling. (If you have never experienced a rollercoaster descent, believe me, it's noticeable ;-). So why and how does it differ for the pilot in the cockpit ?

You seem to assume managing an abnormal situation is natural. It actually requires a lot of training to discard feelings and rely only on instrument indications. Pilots have flown inverted without knowing it, or think they are flying inverted, while they are just flying upright. It can happen at any time, to anyone, when external references are lost, and in particular at night. It's called spatial disorientation. The "the head-up illusion" (p7) for instance can give the feeling of pitching up while the aircraft is accelerating down.

You are making my point, actually. In an elevator you have zero spatial orientation, yet you would know unmistakeably that you are going/falling down.

Your elevator comparison is biased, the elevator is falling at constant acceleration (compatible with our feeling of gravity), the problems arise when this acceleration either changes in direction, or in magnitude. The rollercoaster example is meaningful: Close your eyes, you won't know in which direction your are currently moving (and you will start to feel very bad).

I have been on many rollercoasters. I know when it's going down (eyes open or closed). Especially before the first drop (comparable to AF447's steady cruise flight prior to the stall).

@summerrain I think you're conflating acceleration and velocity. Acceleration is the change in velocity over time. When you're in an object like an elevator, you feel a change in gravity from accelerating up or down. If you go up 100 stories in an elevator, you only feel a change in gravity while the elevator's velocity is changing (ie, starting or stopping). You don't feel anything while it's just moving up or down at a constant velocity.

"AF447's steady cruise flight prior to the stall" -- Have you read the accounts? The aircraft climbed 3000 feet first, while being tossed around by turbulence and by the pilot's overcorrections, then stalled. I've been in a plane that was stalled (for practice), and I've also experienced turbulence; the sensations I recall during the stall were mild compared to some of that turbulence.

On a really tall rollercoaster you might be accelerating rapidly downward for about 300 or 400 feet and then suddenly accelerate rapidly upward. Go ride a rollercoaster 3000 feet tall and see if it feels the same. (Oh wait, you can't do that.)

Fly your pane like a rollercoaster and it will. A ride on the Vomit Comet costs $5k.

@Mazura:  You can do pretty much the same thing on a sailplane (though not as long) for a lot less money.  And in a single engine prop plane, turbulence can give you moments of enough negative G to bang your head on the roof, even though you're belted in.

You only feel the acceleration downward. In roller-coaster this sensation is maximized for maximum thrill. A stall isn't instant: some parts of the wing can be stalled while the rest still provides proper lift. Once the airplane is near or at terminal velocity in a stall it will feel no different from regular straight and level flight.
The onset of the stall was paired with some turbulence which would mask the downward acceleration.
Pilots are also trained to ignore their own sensations because they can be misleading leading to spatial disorientation. You can be in a spiraling dive and still feel as if you are flying straight and level.

The aircraft encountered vertical accelerations between +0.5G and +1.7G.

+1 for mentioning the spiral dive.  In a 60 degree steep turn you can be pulling +2g and not notice

There is also the lack of wind. Plus since the op mentioned elevators, might be worth pointing out you only feel the initial acceleration, after that you only feel vibration .

@summerrain just because they experience momentary accelerations in that range doesn't mean they felt a constant "falling" acceleration.  I don't know what the limit is, but you can be descending at a constant rate and not notice the less-than-1G especially if you are sitting.  Close to zero or negative G's and of course you will notice because you will float, but as long as there is some constant positive G, you might not realize you are falling.

I think this answer is onto the right track by mentioning that acceleration is what is felt, but fails to get into the areas of constant velocity or equilibrium which is really what is masking the sensation of descent.

@2NinerRomeo that's what terminal velocity is about.

Does this answer mean that once a skydiver reaches terminal velocity, they no longer feel as though they are falling?

@ratchetfreak, Yes, Indeed, you are correct.  I retract my statement about failing to mention constant velocity/equilibrium.

@JPhi1618 If you're falling at a constant rate, you feel 1G, regardless of what that rate is. Only acceleration produces G-forces.

You only feel the plunging sensation during the initial downward acceleration.  Once stabilized at a constant rate of descent, things feel normal again.  The other thing is, the amount of vertical acceleration from a stall type maneuver does not result in 0 or negative G, just less than 1. You'll feel getting light in the seat, you won't lift right out and if you are being assaulted by several stimuli at once, as this crew was, you won't even notice it.
You can easily make an airplane go 0 G or negative by getting some speed and pushing hard.  It's called a "bunt". I can make a pencil float in front of my face for about 3-5 seconds in my airplane (push a little bit too hard, and all the crap on the floor ends up on my canopy)  If you are able to nail the 0G point precisely, it's exactly the same sensation as being in a space craft, although short lived.  A jet going 300 kt can keep it up for 30 sec, which is how they train astronauts for weightlessness.

To use the technical term, you feel the jerk more than you feel the acceleration.

You are forgetting one large difference between an aircraft moving through the air (still generating some lift), and an object in free-fall. Constant velocity in a gravity well (such as on the Earth) will cause you to still experience the acceleration due to gravity. It's when you're accelerating that you feel different.
Think of it this way - imagine driving over a hill in a car with cruise control. It's only as you're cresting the hill do you feel "weightless", because you are accelerating downward. On the way down the hill (assuming cruise control maintains speed downhill in this car), you will experience gravity normally.

I don't think it's correct that you feel speed changes only. Falling at a constant rate, you'll notice you are lighter – especially once you start floating :-) as JPhi1618 mentioned

@summerrain That's not true. If you're in a plane falling at a constant rate, you'd feel the exact same weight as normal. What you're probably thinking of is the 'vomit comet' planes, that astronauts use to train in weightlessness. Those planes aren't falling at a constant rate, they're accelerating towards the ground at 9.81 m/s^2. Since everything inside the plane is also accelerating towards the ground at the same rate due to the normal force of gravity, the people inside feel weightless. To be falling at a constant rate, the vertical acceleration would need to be zero.

@summerrain Falling at any constant rate, even zero, means that the forces applied to you in all directions sum to zero. So precisely the same forces on you (those that, including gravity, sum to zero) can mean you are falling at any rate or not falling at all. This is one of the reasons pilots are specifically trained not to try to judge aircraft motion based on subjective force experiences -- flying by the seat of your pants is not encouraged.

@summerrain Wrong. Falling at constant rate you will not only not feel lighter but if you were to weigh yourself on a scale your weight will not be lighter. The scale will only register you as lighter when speed changes. Weight is caused by acceleration, not speed. That's why gravity's physical unit is acceleration (m/s/s) not speed (m/s)

Sensation of motion can come through three of the physical senses.  Each of the three senses is successively unavailable as we move through the examples of an amusement park ride, the plummeting elevator, and AF447.
Vision
This sense is fairly well understood, so I'll limit this bullet point to pointing out that the visual sense is correlated in the human brain to the vestibular sense. In the amusement park ride, the inputs recieved from the vestibular sense and vision match, and there is no resulting confusion about the perceived motion.  In cases where those two senses are decoupled, it is common for nausea to result.  In the elevator and airplane in instrument conditions, a visual sense grounding you to a world reference frame is denied.  This is one of the senses which can detect unaccelerated motion, more on "unaccelerated" below.
Touch/Kinesthetic
Here we have information which comes in through our sense of touch, primarily through nerve endings in the skin.  The roller coaster comes through here again by exposing you to the elements giving you a "rush of wind" in your face as you move through what may be a stationary mass of air.  In the closed environments of the elevator and cockpit, this is again denied.
Vestibular/Inertial
Here is where things get interesting. For the purposes of this discussion, I'll add to the vestibular senses the "seat of the pants", "pit of the stomach" and other bodily means of detecting acceleration.  Several other answers have pointed out that all of these senses are sensitive not to velocity but to acceleration, which high school physics reminds us is the the time rate of change, or derivative, of velocity.  This means that if velocity is not changing we are not going to have a sensation of motion, although in a gravity field, we may have some sense of orientation (you can have a sense of hanging upside down or laying sideways).  There is no difference to these senses between sitting on the ground with the engines running, cruising at 500 knots in level flight, or being in a 500,1000, or 3000 foot per minute descent.  As long as velocity is not changing, these senses cannot tell the difference.  This is the problem with AF447, No visual, no touch, and in unaccelerated descent, no vestibular sense.
If the airdata instruments are offline, there is nothing to indicate the descent until the cabin pressure starts to increase, putting some pressure on your eardrums.  That also won't happen until the aircraft descends to a point where the outside air pressure increases to a point above the cabin pressure then continues to build to a point that you notice.  They were doomed.
There is another problem, even with detecting acceleration. The problem is connected to the human brain's remarkable ability to filter out stimuli.  A normal human brain is capable of adjusting to the tint of your sunglasses, ignoring conversations in a crowded room, and is very good at not notifying the conscious thought center of the pressure your elastic socks are putting on your ankles.  Indeed, it can even lose track of constant accelerations. If you live near a playa, you can do a blind test by driving your car out and enter a gentle turn giving a tenth of a "g" of lateral acceleration. Have someone else do the driving.  I am here to tell you that the resulting sensation can be filtered out.  Even our sense of acceleration can be lost under constant acceleration.  In physics, there is yet another level of derivative, the time rate of change of acceleration, called "jerk", which is harder to not detect.
For more information on the brain's filtering capability, look up the "Reticular Activating System".  Understanding it can help you be more safe, and also help you reach your goals.

A few things.
That accident took place at night and in IMC while the aircraft was passing through a storm which I suspect had quite a bit of turbulence in its core.  This would most likely have caused enough vestibular disturbance for the crew to make the acceleration towards the deck go unnoticeable.
Secondly the crew was on an IFR flight which means flying by instruments in conditions and specifically trained to rely on cockpit instruments for aircraft orientation and not vestibular senses.  Doing otherwise will get you killed in any other situation.  Once the aircraft reached its peak descent rate in dark night over water and with the air data sensors iced over, preventing the flight control computers from receiving accurate air data, there was no perceptible means for the crew to sense they were in danger until it was all over.
Pilots do experience the vestibular sense of accelerating as occupants in a carnival ride do - try flying aerobatics some time and yes, it’s quite noticeable when pulling negative Gs, etc.  it’s just not a good standard to use to judge movement in an airplane by and without visual references outside of the aircraft it is difficult and very dangerous to use vestibular senses to accurately assess aircraft attitude from.


text:
Has turbulence ever killed a passenger?
The question is on my mind, because many passengers think there is no need to wear the seatbelt when the seatbelt sign is not illuminated.

Related: Do we have a unit to measure turbulence? (which links to this table  of effects) and How dangerous was the turbulence on this commercial flight?

I was on the 1996 Air France flight. A terrifying memory to this day. I was sitting close to the man who died some weeks later. I believe as a result of this accident that many airlines insist on seatbelts at night, particularly when crossing the equator.

The following is a list of deadly turbulence accidents on jet airliners since 1980.
Excluded are accidents involving jet airliners designed or built in the former Soviet Union.
For a more complete list see aviation-safety.net.

10 May 1980, Indian Airlines B737-200, near Rampur Hat/India:
The aircraft experienced severe en route turbulence. 2 of the 132 passengers were killed.
7 October 1981, NLM CityHopper F28-4000, Moerdijk/Netherlands:
Shortly after takeoff, the aircraft entered an area of severe thunderstorm activity. The aircraft apparently had a catastrophic in flight structural failure due to severe turbulence because it was seen to emerge from the clouds with one of its wings broken away. All 4 crew members and 13 passengers were killed.
16 August 1982, China Airlines B747, near Hong Kong:
The aircraft encountered severe in flight turbulence. 2 (out of 292) passengers not wearing seatbelts were killed.
3 October 1990, Eastern Air Lines Flight 791, DC9-31, Florida/USA:
The aircraft clear air turbulence at 31,000 feet (9450 m). 1 injured passenger died 3 weeks later.
5 September 1996, Air France B747-400, near Ouagadougou/Burkina Faso:
Severe turbulence associated with a weather front seriously injured 3 of the 206 passengers. 1 of the 3 passengers later died of injuries caused by an in flight entertainment screen.
28 December 1997, United Airlines Flight 826, B747-100, over Pacific Ocean near Japan:
The aircraft encountered severe turbulence during cruise about 2 hours after departing Japan. 1 of the 346 passengers was killed and 3 crew members sustained serious injuries.

I don't think the last one should count.  The question relates to pax killed by being rattled around the cabin. That was just the pilot flying stomping on the rudder in concert with aileron inputs, as if he was in a light aircraft, which is a really bad thing to do in a swept wing jet.  Vertical fins are not designed to withstand the bending from full rudder inputs above certain speeds, and in a swept wing jet with yaw dampers you don't need to actually touch the rudder pedals once airborne unless an engine quits.

@JohnK: You are right. I must have misunderstood my own question ;) I'll remove AA587.

Interesting tidbit: the second entry on the list involved the aircraft inadvertently flying through a tornado.

@Vikki It's the tornado that broke the plane, providing "+6.8 G and -3,2 G" stress to the airframe... I didn't think "turbulence" would cause structural failures on a plane in this day and age.


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What’s the speed for lowest sink rate on a DA20?
In the DA20, the POH lists for glide speed (flaps up) only one speed giving the best glide angle / furthest distance (73 KIAS).
But what about, if I don’t need that much distance to glide (e.g. because I’m high above an airport) and rather want to optimize for the maximum time airborne, i.e. the lowest sink rate. Which speed would I need to fly for that? I couldn’t find any L/D diagram in the POH either where I could find it from.

Are Vx and Vy given in POH?

@CptReynolds yes, Vx 68, Vy 75 (both KIAS)

I asked the manufacturer and they said, pilots should just fly best glide speed, because of the drag of the windmilling prop that's the best recommendable procedure. They also told me that they don't publish the actual best sinkrate speed nor the L/D diagram since regulations don't require them to.

Admittedly, my prop performance theory is a tad rusty, so maybe someone else can provide proof here.
I seem to recall that - assuming a quadratic drag polar which might not hold true for your aircraft - minimum sink speed has a theoretical value of .76 x best glide speed, so in your case that would be around 56 KIAS.
If you’re comfortable and consider it safe to do so, why don’t you (at a safe training altitude) trim for 73 KIAS and establish a stable idle descent, note vertical speed descending through a predetermined altitude, reposition to initial altitude and repeat in maybe 5kts decrements down to stall speed or some safe margin above - the former of course only if flying right down to stall is still trained and allowed in small aircraft. This will give you at least a rough idea of where minimum sink speed lies and what rates to expect.
The DA 20 Approved Flight Manual doesn’t help much, by the way.

Never pictured Vx and Vy as a min sink rate and Vbg diagram turned sideways by adding power. That min sink rate curve for gliders seems to be a combination of the linear AOA / Coefficient of Lift graph and the lift / square of speed graph.  That would put the maxima a little further away from stall than I had imagined. Looking at old stuff a new way, thanks!

Thanks for this idea how to find it, I might do it next time I have the possibility to do so. And regarding that the AFM doesn't help: The manufacturer's answer to that is that an L/D diagram or the like isn't mandated by regulations and thus won't be published

The DA 20 Katana is a powered aircraft, but the rate of descent vs airspeed graph from
gliders still applies.  Generally, the lowest rate of descent (vertically) is slightly lower than Vbg.  This makes sense as higher AOA will generate more lift at the expense of increased drag, which slows forward motion.  Vertical component grows larger, allowing lower speed, horizontal vector grows smaller, therefor less distance covered.
The relationship holds all the way down to near the stall point, any slower, Coefficient of Lift begins to decline and sink rate increases.  A little more dangerous to fly here, as a gust of wind or a turn could lead to a stall.
If you are high over the airport and approaching to land (getting near the ground), "going slow" or being on "the back side of the power curve" is not as safe.  Better to hold speed and drop flaps.
But if you are higher and wish to try minimum rate of descent (flaps up or down), "real" numbers for your particular plane can be had.  Keep that carb heat on too, if needed.

Minimum rate of descent would probably be closer to Vx.


text:
which country’s requirements for higher license?
If I have a license registered in country A and want to add a higher license with a flight school in country B (both countries are in EASA), which country’s rules apply for requirements for the higher license? Let’s say, country A requires 10 more hours of something for the higher license, does that also apply if the higher license is done in country B?


text:
Does the responsibility for the plane shift from the PIC, if they leave the plane?
Is the PIC still responsible for everything when they leave the plane and a co-pilot is staying on board? Are there differences between operational reasons for leaving the plane (doing the walk-around) vs private ones (going to grab a snack in the airport during a layover)? Does the PIC have to specify someone to be responsible for the plane while they’re gone?
If there is no international rule but airlines have rules in their SOP, what are those rules generally?

If you aren't controlling the aircraft, PIC doesn't really have any meaning at all. There is no reason to identify anybody as PIC if you aren't taxiing, flying, or about to push back... So I think you are asking who has legal responsibility for the aircraft while it is parked? That is much different than PIC.

@RonBeyer A potential situation: captain is outside doing a walk-around while boarding and smoke comes into the plane causing a passenger evacuation. What's the captain's responsibility in this case? I'd imagine it would be similar to the case where the captain is sleeping in the back and something goes wrong.

@user71659 That doesn't have anything to do with PIC though, and I don't know of any airline that doesn't have at least one pilot on board at all times while passengers are aboard. Captain and PIC are two very different things. A captain can't be PIC and be "sleeping in the back" at the same time. The problem is that the question supposes that a pilot must be responsible for an aircraft at all times, which isn't the case, for example if an empty plane is being towed and has an accident, no pilot could be held responsible...

@RonBeyer "A captain can't be PIC and be "sleeping in the back" at the same time." Incorrect. The PIC remains the PIC even if they are on a rest break or in the bathroom (and can log all this time). There was a related question. You're asking a different question as well. "Is there a PIC all the time in an airplane? No (towing)" is a different question from "Can a PIC be held responsible for something that occurs while they are outside?"


text:
Are there Instrument-only airports?
Are there airports that in general don’t allow VFR traffic and e.g. don’t even have a VFR chart published and just allow IFR traffic?

I doubt it. London Heathrow technically does allow VFR traffic (although I doubt they have any). And if VFR is allowed at Heathrow, I don't think it is banned elsewhere.

No.
Think about what you are asking.
Scenario:
You're a captain at the pointy end.  All your instruments have failed.  Maybe let's add in some extra emergency element for good measure.
You need to land at the nearest usable airfield using little more than the good old Mk1 Eyeball.
Your concept of an "IFR only" airfield can shout "IFR only" down the radio all they like until they are blue in the face.  But in line with the rules of the air, the captain has the ultimate authority, if he's got clear VFR, he'll be landing there as he has no other options.
Keeping the spam-can brigade out of the way of busy airfields is not the job of airfield policy, its the job of airspace policy. And that works perfectly well.
Conclusion:
Even if they might be rarely used on a daily basis, having VFR procedures in place is important whether you are a major airfield or some grass strip.
That way, everyone knows what is expected of everyone else.

"All your instruments have failed." That is already a bona-fide emergency situation, likely even in VFR. No need to add anything extra; that plane needs to land, and the captain has likely declared an emergency well before reaching the airfield. It's bad enough if just one of your instruments have failed. Try, in mid-air, placarding your altimeter, airspeed indicator and variometer, and land on an unfamiliar airfield. Congratulations; about the only thing you lost was the pitot-static system. Now they aren't placarded, but doing something like spinning wildly all over the place. Good luck.

My "scenario" was never meant to hold up to deep scrutiny, and indeed perhaps not the best example in this context, but the captain would still technically be landing VFR since he has nothing else.

There are I think 7 or 8 Major US airports where you have to have an IFR reservation to land.  Here is info on landing for DCA - Ronald Reagan National Airport and LGA - LaGuardia Airport for example http://www.fly.faa.gov/ecvrs/  And as noted, in an emergency ATC will step in & help.

@CrossRoads I don't know about the US, but I did check my airport directory for Stockholm/Arlanda (ESSA) which is Sweden's largest airport. That lists prior permission required for all VFR traffic; slot assignment required; and in case of radio failure, to leave the airport's airspace and land on a different airport. So VFR is allowed, but they really don't want aircraft out of communication wedging themselves in between others. Göteborg/Landvetter ESGG, probably the #2 airport in Sweden by movements, appears much more relaxed in this regard, though I suspect they wouldn't be happy either.


text:
What’s the steepest possible descent for big commercial jets?
What’s the highest descent rate, an A320/737/A380/younameit can achieve in safe controlled flight and how do they achieve it? E.g. can those jets do side slips to have a steep approach angle (maybe not for final) if they need to land ASAP, for example for a medical emergency / rapid descent due to pressurization problems?


text:
What are the requirements to become a language proficiency examiner?
What are the requirements to become a language proficiency examiner? Does the person have to be a pilot? Does it have to be a flight instructor?
What kind of language proficiency rating do they have to hold to test level 4 / 5 / 6? I guess they have to have 5 to test 4 and 6 to test 5 and 6, is that true?

A what? Never heard of of it? Please show a regulations that requires one?

@jwzumwalt The FAA had not adopted the ICAO English language requirements, so I suspect that's why you haven't heard of it.

The answer to this question depends entirely on the country. Some have no requirements for the training, others like the UK require the training organization to be approved by the national civil aviation authority,

@JuanJimenez could you expand that with a bit more details into an answer? That's basically what I'm looking for, maybe some links about it as well or some hints where I find more details or so

I need to know which country you're referring to before I can provide an answer.


text:
Could a airship be built using vacuum as the lighter-than-air “material”?
Could an airship be built that uses instead of a very light gas a vacuum to keep it floating? Or would the materials needed to keep its volume even without the pressure from the inside always be heavier than the lift by the vacuum could handle?

Sure, if materials science is no object.

Maybe some day someone will develop an envelope made of graphene or something that is light enough and able to sustain the 14 psi implosion pressure.

Vacuum is much cheaper than helium, vacuum can also be manufactured on the launch site or onboard the airship. It is really a pity that it is impossible to make a  hull that is lighter than the helium fill that we are replacing and can withstand the external pressure.

@Penguin this is not how physics works. Vacuum is lighter than any material. By definition. Because it has no mass.

@Florian. No, the mistake I made was understanding the relationship between weight and buoyancy, I assumed a vacuum created no  buoyancy force, but that is wrong.


text:
"right traffic pattern" vs. "right-hand traffic pattern"
Is it officially called "right traffic pattern" or "right hand traffic pattern"? I'm mostly interested in EASA terminology, but also FAA / other areas of the world.
Background: After I always said "right-hand traffic pattern" on frequency I got feedback from other pilots that I should leave out the word "-hand" to keep communication on frequency on a minimum - so I was now wondering, if there is any official terminology for that to be used. When looking in the Eurocontrol Phraseology Database, I only found "right hand pattern" for holding patterns.

Leaving out "hand" is getting a little anal.  If you are descending into a terminal control area in a jet where the chatter is non stop and you have to wait for a chance to break in, yes you economize on words but at a VFR airport?  Fun fact: In Commonwealth countries we use "Circuit" instead of pattern.

"Right traffic pattern" and "right hand traffic pattern" are one in the same.  Usually on the radio either a controller or a pilot will abbreviate it to simply "right traffic" to expedite radio communications.
For example, A C-152 student pilot is inbound for a controlled airport and says something like "Plainview Tower, Cessna one two five X-ray ten miles to the south at two thousand, inbound with ATIS Golf."  If a controller wants a student pilot to fly a right traffic pattern to land on runway 16 which is active, they might say something like "Cessna one two five X-ray enter right downwind runway 16.  Report abeam the tower."  The pilot then responds reading back the instructions to the controller and might say "right downwind one six.  Report abeam.  Two five X-ray."


text:
How do cruise missiles stay aloft?
How do cruise missiles fly? Where does the lift come from? On pictures you can see that they have wings what are tiny compared to the body / payload. Do they generate enough lift? Or is the propulsion, other than in most aircrafts, not just used to overcome drag but also to generate lift, i.e. the thrust force is angled down?

I'm not sure this is a dup. The other question insists pn wing load whereas this one focus on lifting body and the vertival component of thrust.

@ManuH: Both are speculative and missing in real life. The other answer should be pasted here.

Many cruise missiles have wings of some sort, although these can be very small like on the P-700 (barely larger than their control surfaces) or foldable in such a way that makes them not obvious when the missile is in a stowed configuration, like on the Tomahawk or Kh-55.
Additionally, the narrow body of the missile provides lift. Though it is much less effective at it than a wing would be, this is a design trade-off that can be worth it in some scenarios. For supersonic cruise missiles the narrow-body lift is usually enough and the lack of wings is a plus, as they would generate a disproportionate amount of drag at $M_{\infty} \geq 1.0$.
One of the main traits of slender wings (and bodies) is that their effect on the surrounding airflow grows with speed at a much  slower rate than that of a high-aspect-ratio wings, making them suitable for high speed applications.

Few of the cruise missiles I can find - and none of the ones you mention - appear to have any lifting body dimensions. It really has to do with the fact that the high speed and high wing loading mean that only small wing surfaces are required.

The central body (engine and warhead housing) of the missile provides lift, as even a small angle of attack is enough to generate transversal force. If you want to run the numbers, use the diameter and length for slender body lift.


text:
Is VOR identification automated in modern avionics?
Is the VOR identification via audible Morse code automated in modern autopilots / avionics?
Or does a pilot in an A380 still sits in there, listens and compares the .-. .. -.. to her charts?

Related

Modern air transport and bizjet avionics do decode the audio ID and provide it digitally on the data output bus. This data is displayed on the Nav display or RMI (depending on Mode) along with the frequency. It's not as common for general aviation radios to have this capability.
VORs that comply with ARINC Characteristic 711, VOR MB Receiver, output the ground station ID in two ARINC 429 Labels.  The first and second characters will be in Label 242.  The third and fourth (if there is a fourth) characters will be in Label 244.  Characters are coded in ISO 5 format.
Since all modern radios are DSP based, detecting and decoding the audio tones is a fairly straightforward process.  The radios also provide an analog audio output, so the crew can select the VOR radio on the audio panel and listen to the Morse code if they desire. (I'd be surprised if you found a pilot that did that routinely.)
ILS is similar. The ILS ID is contained in Labels 263 and 264.

To add to this excellent answer; it's becoming increasingly common for VOR stations to broadcast an audio voice signal encoded into the navigation frequency. With the right radio this will allow the pilot to hear a voice reading out the ID, rather than the Morse code. Much handier for those who can't decode Morse in our heads!

Even small planes have radios that can decode & display identifiers. The Avidyne IFD540 is one such example.

As everyone said, modern avionics is capable to decode the audio code of the station, even on small GA airplanes. This is how it looks on the G1000 PFD/MFD (in the top left corner of the screen; taken from the Garmin manual):


text:
Does the flight crew in big airliners hear the cabin crew PAs?
In big airliners, does the flight crew hear the PAs the cabin crew is doing to the passengers, e.g. on their headsets? If not, how do they make sure they’re not talking simultaneously to the passengers?

Yes, when the pilot makes the selection to speak on the PA, he will hear if anyone else is using the PA. They would just wait until the other party is finished, or they can take priority and speak over them.

Some commercial aircraft can be equipped with an option to indicate to the flight crew that Public Address (PA) is currently in use (at least this option exists in the Airbus A330/A340 families, I am not sure for other series). In the A330/A340, the ECAM Memo section of the Engine and Warning Display will show a message "PA IN USE" in green color during cabin announcements.
In addition to the other answer by Mike Sowsun, pilots can not only listen to cabin PA announcements when they want to do one themselves, but basically anytime they want to monitor PA, by selecting the PA reception knob on their Audio Control Panel (ACP).

Most of the time though, this will be deselected because it is simply not necessary for the pilots. However, on the A330, it is usually intended that the third ACP (the one that someone in the observer seat would use) has the PA reception selected, such that the Cockpit Voice Recorder can record any cabin announcements on the PA.
All of this information as well as the image can be found in online resources like the A330 Communications and A330 Indicating Recording Systems sections accessible on Smartcockpit.


text:
Why did Air India 139 fly so far south?

(twitter.com)
An Air India flight from Delhi to Tel Aviv started very far to the south before turning west. Why did it do that? Which airspace did they have to circumvent for political reasons? The India-Pakistan border is way further up north, so that shouldn't be the issue, or is it?

I remember from about 5 years ago is that there is only one air corridor for commercial jets passing thru Iran airspace, making it quite congested and slot critical. Overflight costs is also very expensive.
Also the perception is that the airspace over Iran is not the safest in the world. I believe most airlines fly to the north or to the south of Iran, unless flying to a destination in the country of course. Example MH17, even though a lot of airlines were flying over the Ukraine, MH was criticized for flying thru a "war zone".

Well, a flight to TLV won't go soon through Iran airspace anyways. I was wondering about how far south it went inside India before turning towards the ocean.


text:
Is there any combination of two airports that are connected via taxiways?
Are there any 2 (or more) airports that are ground-connected so that e.g. aircraft can taxi between them? Would that be even possible from a definition point of view, having a common area with two airports?
I thought of airport being a facility with an ICAO identifier, but maybe a different definition suits better here?
I could imagine e.g. having an international one and a GA / regional airport being so close together that they were connected at some point. Does anything like that exist?
Update: I'm also curious about airports that are not directly connected in a physical way but e.g. in an operational way: an airside bus takes passengers from the gate in a terminal in airport A and brings them to their flight in airport B or other situations like that.

With some aircraft, all roads are taxiways. :)

Does the Polderbaan and the rest of Amsterdam Schiphol count?

Would historical examples count?

wow, @TannerSwett, 2 things come to mind when I see that. 1) rear visibility (when driving) is just a concept, not a reality (not that most people look before changing lanes). 2) a minor fender bender, which is likely when people are gawking at you, is liable to be very expensive to repair and a major pain if you're on vacation at the beach and flying home is your plan.

Do you count "sharing a control tower" as "operationally connected"?

@Mark shared tower but physically different airport? That sounds interesting! Where is that?

@Florian, not fully shared, but Spokane International's tower also handles approach control for Fairchild Air Force Base, 5km to the west, presumably due to Spokane International's runway 25 departure routes intersecting Fairchild's runway 23 arrival routes.  Fairchild ground control and departures are handled by Fairchild's tower.

@TannerSwett For some aircraft, roads are runways (English CC available).

There are numerous joint use (commercial/military) airports with separate terminals/ramp areas. There aren't many where there are two connected fields. The only one I'm aware of is Naval Air Station Whiting Field near Milton FL that has two operational fields with separate identifiers.
There is NAS Whiting Field - North. ICAO: KNSE and
NAS Whiting Field - South. ICAO: KNDZ
The two fields have a central support base area between the two fields. There is a single taxiway that connects the two fields around the east side of the support area.

I think Chitose and New Chitose on Japan's Hokkaido island fit the criteria.
One is an air base (Chitose) with ICAO code RJCJ, while the other is the island's primary civilian airport (New Chitose) with ICAO code RJCC. They have separate runways, but they're jointly operated and linked by taxiways.
It's quite cool watching the air base's jets while you're waiting for your flight.

Image licence details

Interesting airport! Do they have two separate ICAO identifiers, by chance?  Nice find, and welcome to Av.SE!

@Ralph They do: RJCJ for Chitose and RJCC for New Chitose. Thanks. I probably won't post here often, but I saw this question and immediately went "hey, wasn't New Chitose like that?"

Very cool - wish I could upvote this answer twice, since is exactly what the OP was asking about, as well as an interesting & unique combination.

Wow! Thank you for the bounty rep :-o :-)

The airports in Berlin share runways.
Strictly speaking, one of them (BER) isn't really operational... But it will be. Soon. For sure... Hey, look over there: a monkey with two heads!!

google maps

I'm not following your comment about a monkey with two heads...

@Michael, Berlin Brandenburg has been missing construction deadlines for almost a decade now, and management has been trying to distract people from that fact.  (Incidentally, they might have been more successful with a three-headed monkey.)

IIRC, SXF will be closed the day BER goes operational, and will then be rebuilt into the BER freight terminal.

@Michael reference to Monkey Island http://monkeyisland.wikia.com/wiki/Three_Headed_Monkey

Oakland International Airport (IATA: OAK, ICAO: KOAK, FAA LID: OAK) is an international airport in Oakland, California, United States.
Oakland International Airport covers 2,600 acres (1,100 ha) and has four runways.

OAK has in essence, two independent operations running as one complex. Commercial flights and cargo operations operate primarily out of the South field complex and general aviation, charters, flying clubs, and FBOs operating out of the North Field.

South Field (Commercial and cargo operations) (one runway):

Runway 12/30: 10,520 ft × 150 ft (3,206 m × 46 m) asphalt

North Field (general aviation operations) (three runways):

Runway 10R/28L: 6,213 ft × 150 ft (1,894 m × 46 m) asphalt

Runway 10L/28R: 5,458 ft × 150 ft (1,664 m × 46 m) asphalt

Runway 15/33: 3,376 ft × 75 ft (1,029 m × 23 m) asphalt

There is a connecting taxiway that will take you from the middle of 12/30 to 28L, over the Ron Cowan Parkway, but my experience is that no commercial flights use the North Field.
https://en.wikipedia.org/wiki/Oakland_International_Airport

"my experience is that no commercial flights use the North Field." Given the maximum 6,200 ft. runway, yeah, I would imagine that they don't. I wouldn't be surprised if some GA (say, private jets) use 12/30 sometimes, though.

I recall they used North Field to land jets when they had to close runway 12-30 for rehabilitation. They took off on taxiway W. The additional noise made them give out tickets to residents for a vacation.

Someone built a housing development directly north of all three runways at OAK. The houses are well insulated and have little noise as a plane flies overhead. Unless you leave a door open. Trivia fact: directly north of the airport is the HQ of the Oakland Raiders football team. It's thankfully out of the flight path.

This is a historical example, and doesn't quite fit your definition of "airports," but it may be of interest that during World War II, the US and Canada built makeshift runways on opposite sides of their shared border, or perhaps a single runway across the border, only half of which was used at a time. The purpose was so that US-built airplanes could be delivered to Canada while remaining compliant with US neutrality laws. Planes piloted by American pilots would land on the US side, be towed to the Canadian side by civilians, and then take off again under the command of Canadian pilots. See the Canadian Aviation Historical Society for details.

One strange case of airports connected in an operational but non-physical way: Allentown (Lehigh Valley International)—ABE, Pennsylvania, USA and Newark—EWR, New Jersey.
United Airlines connects these two airports by bus (they're about 80 miles apart). Passengers booked on these "flights" can go to the Allentown airport, check in, board a bus, and be taken to Newark for their connection (they'll go through TSA security at Newark, but their checked bags will be transferred, so there's no need to go to the bag drop/ticket counter). From Newark, they board the bus from inside the secure area of the airport terminal, just as if it's a plane, with luggage transferred for you (they don't drop you inside the secure area at Allentown though, it's not a true airside-to-airside service).
Passengers even earn frequent flyer miles for the bus ride.

The "legacy" Continental Airlines (now part of United Air Lines) also had connecting FLIGHTS from Ellington Field (EFD) to IAH/KIAH (George Bush Intercontinental) back in the 1990s. About 30 statute miles distance.  They used ATR-42s or similar, IIRC.  But, you had to have a ticket from IAH to somewhere else in order to fly EFD-IAH.

Taiwan's Taoyuan International Airport (RCTP) is connected by a taxiway to Taoyuan Air Base (RCGM) just southeast of it.

Please don't just use IATA codes. They're easy to typo, easy to misunderstand and easy to not understand at all. Without clicking on the link, I have no idea even what continent you're talking about

@DavidRicherby "easy to not understand at all" - given that IATA code is what most people use when booking tickets I'd say it's more recognizable than the ICAO code.

IATA codes are recognizable as being IATA codes. But if you see an IATA code that you're not familiar with, you can't understand anything at all about it, other than "This means some airport somewhere in the world." And note that I said don't just use IATA codes, not don't use them at all.

@Steve Kuo: But this is the AVIATION site, not the travel site.  Most of us fly planes far more often than we book tickets.  (And on the few occasions where I have booked commercial flights, I just said "London" or "Helsinki" without ever thinking about airport codes.)

@SteveKuo, quick - what airport is "JRN"?  How about "SWJU"?

Regarding "airports that are not directly connected in a physical way but e.g. in an operational way": the two main Paris airports (Charles de Gaulle Airport/CDG/LFPG and Orly Airport/ORY/LFPO) have a bus shuttle to transfer people between both airports. This is done because domestic flights are served via Orly while international flights are served via CDG.
So if you were to fly from Berlin via Paris to Martinique (french overseas territory), you would arrive in CDG, take the bus to Orly, and take the plane to Martinique.
The airports are several kilometers apart, though. Also, the buses depart outside of the airport, rather than at the gate.

That's a good example. It's also got to be a major pain for the traveler and a logistical nightmare for the operations groups!

and welcome to [Aviation.se]!

Would be interesting to know if there are two airports that have shuttles airside, so before immigration / customs / hand luggage security etc, so basically while being in transfer still. And even maybe checked in luggage transfer.

The CDG/Orly bus is landside, though, right? Not airside? CDG isn't even connected to itself airside, let alone to Orly. Even within terminal 2E, there are multiple separate airsides (which is not a particularly passenger-friendly design.)

FWIW, there's also a bus between Dubai (DXB) and Abu Dhabi (AUH) as I recall, but that one is also landside as far as I know.

If you start with land-side busses, many airports are connected. And don't forget the trains running between Dusseldorf Airport and Frankfurt Airport, landside but with an LH flight number... https://en.wikipedia.org/wiki/Lufthansa_Airport_Express

@reirab Expecting passenger-friendly design from CDG sounds like a venture doomed to fail miserably.

@JanusBahsJacquet You're not wrong.

A former example is Rhein-Main Air Base (ex-FRF) and Frankfurt Airport (FRA).  The main runways are roughly east-west, with the airport to the north and the former air base to the south.
The base closed in 2005 and the site is now the FRA cargo terminal, with a third passenger terminal being built there, too.

Anchorage’s Ted Stevens International PANC ANC is connected by a taxiway to Lake Hood Seaplane Base PALH LHD. Lake Hood also has a gravel landing strip which is not part of ANC.

Yes, in fact, there are many: just take a look at this map

On this extract you can see one of many distinct airports connected by taxiways (here La Grande 4/Lac de la falaise and Lac Polaris)
...
If you are willing to include sailplane and rivers as taxiway.

In Germany, in Schwäbisch Hall there are two airports: "Schwäbisch Hall-Weckrieden" and "Adolf Würth Airport Schwäbisch Hall" (EDTX and EDTY) that are directly connected with one taxiway that even crosses a public road that has to be closed if someone actually wanted to taxi from one to the other airport.
I've landed there and walked from one airport to the other to get a cup of coffee, watched the sky divers depart on the northern airport and then walked back to the southern airport. So yes, that exists!

Scott AFB/Mid-America Airport (BLV/KBLV) fits this criteria.  The west half is a US Air Force base and the east runway and ramp are a civilian/commercial airport.  Notice how long that lone taxiway is that connects the two.


text:
Is there a time limit after the ATPL theory exam to get the license?
If I did the ATPL theory exam now, would I have a time limit to finish the ATPL? If so, what is it?
And what about other EASA licenses, CPL, (E-/CB-)IR, etc.?

In Canada it's two years for the ATPL, but if you obtain a transport category type rating within that period it's indefinite as long as the type rating is current.

Generally you have 7 years for your ATPL theoretical knowledge, but actually it depends whether you already have Instrument Rating and Commercial Pilot License. If you do not have CPL and/or IR, you have only 36 months. If you have IR, you have 7 years.
Moreover, according to FCL.025(c), you have to pass your IR in 36 months after the theoretical exam (nevertheless, IR or ATPL), and then, if those exams were ATPL (I believe that you are going to pass ATPL without separate IR and/or CPL theoretical exams), your theory will remain valid 7 years from the date of your IR.
For more information you can refer this document, part FCL.025(c)(1) and FCL.025(c)(2).


text:
What's preventing aircraft manufacturers to record every single possible value in FDRs?
The preliminary report of the ET302 crash included some data from the Flight Data Recorder. However, the position of the trim cutoff switches is apparently not among the recorded values.
Given that storage is so incredibly cheap nowadays compared to the price range of an aircraft, why don't manufacturers decide to record every single possible sensor value (including all switches in the cockpit) into Flight Data Recorders? Is this a purely economic decision to not do too much more than required by regulations? Or is there some other reason for it?

Related: especially the first paragraph

@freeman that comes down in the end as well to “economic reasons”. But it’s a good point!

@FreeMan as there's currently no good answer, do you want to write one with a few more sentences than just that link so there's one I can accept?

I'm not an expert on FDRs, but I do have experience of hardware and software development in a regulated market (medical devices), so might be able to offer some insight.  Here are a few considerations that occur to me:

Development cost:  Adding storage to a FDR isn't like installing new DIMMs in your workstation.  As well as the design work, the changed device must undergo new verification and validation cycles, the supply of the new components needs to be assured, and the manufacturer must continue to support existing products as well as a new one.  The V&V may well include accelerated-life testing and other expensive activities; it's unlikely to be just a few days' work.
Customer demand: Aircraft operators require an FDR that satisfies regulatory requirements; it's a "tick-box item".  Consequently, that's the requirement that aircraft manufacturers demand of their FDR suppliers.  If there's no demand for extra storage (either for an extended loop time or for a greater number of sensor inputs), then there's no money to be made by offering that as a feature.
Interface limits: I don't know enough about FDRs specifically, but the input interfaces may well have a bandwidth limit which precludes providing more readings without providing further connectors (which then has a big impact on development costs; see above).

And yet many FDRs do record many more parameters than required by regulations - even the newer regulations only require a few hundred parameters, while many newer FDRs record thousands of parameters.

Fair enough (I said I'm not a domain expert).  I think my observations stand, but are probably not the whole story.

Monitoring should not slow down systems updating data and data transmission between systems. Data storage is easy, but monitoring is expensive it might increase wiring, and overall load

Because storage is the easiest thing to add. What you think should be easy actually requires monitoring for every single switch and device, which would add a lot of weight and complexity. In aircraft accident investigations, switches don't usually move from their positions during and after a crash. That is why there is no value in adding that to the FDR dataset.

Your 1st sentence is missing its main part. The 2nd sentence is wrong for any switches which are evaluated by digital hardware. 3rd sentence: in many crash investigations you cannot even find the switches any more.

@bogl And, in many cases, the "switch" is an option on a display screen menu.

You two need to look at a real 737 Max cockpit. You have no idea what you are talking about...
I have worked in avionics for 40 years. You? httpss://www.straitstimes.com/world/united-states/us-pilots-complained-at-least-5-times-about-boeing-737-max-problems-records-show

@JuanJimenez Which of my statements is wrong?

@bogl All of them.

Are you serious?


text:
What is an "unsafe airline"?
Question:
Is there any internationally accepted definition for what an "unsafe airline" is based on objective criteria (e.g. fatalities per million passengers) ?
Or is there an international standard or worldwide blacklist ?

All I was able to find are regional lists:

the list of air carriers banned in the EU, AKA the EU Air Safety List and
the International Aviation Safety Assessment for the US.

I'm hoping to receive answers in a similar vein as those for the question What is a “Major Airline”?.

There is no international blacklist of unsafe airlines. An airline can only exist if at least one country believes it is safe. Those regional blacklists are about the best you'll get.
In a formal sense, calling an airline "safe" is generally avoided. A regulator would use a term like "meets safety standards". This might be a small distinction but it helps make it clear that no airline flight is guaranteed to be safe. So the closest legal definition of a safe airline is one that has been certified as meeting safety standards by an ICAO compliant country. However this is not that helpful as different countries have different standards, as proven by the EU and FAA blacklists.
So to get an international comparison for airline safety you need to look at informal sources, which is subject to opinion. But this opinion can be formed by combining multiple official sources. A good example of how this can be done is on https://www.airlineratings.com/safety-rating-criteria/. This criteria is informal and doesn't consider the the most important behind the scenes information, but it is probably the best safety indicator we have.


text:
What is CBIR & Basic IR?
I know that the IR - Instrument rating - is the full-blown option to be allowed to fly IFR. I also know that EIR - En-route Instrument rating - is an option to have a license that's restricted to fly IFR except for during takeoff and landing.
What is CBIR and what is Basic IR? I also heard that Basic IR will "come soon" and "replace EIR". What's the roadmap for EASA instrument ratings in the next years?

The Competency Based Instrument Rating (CBIR) allows you to gain an IR in less hours than the "full" course, by either crediting previous experience or licenses.
I held a UK issued IR(R) (old IMC rating), which meant that I had approx 60 hours IFR time when starting my IR. A full IR would have meant 45 hours training, whereas the CBIR allowed me to do just 15 hours (in accordance with EASA Part FCL, Appendix 6).
The BIR, as far as I am aware, is effectively going to mirror the UK IR(R) but make it a full EASA rating. That isn't confirmed however, as I haven't yet seen any official documentation from EASA.


text:
What did a maintenance person put into the engines after landing?
I was recently flying on a A320-251N and directly after landing & coming to a halt at the gate, a maintenance person approached the engines carrying a box of something that looked like paint cans, opened some lid on the side and apparently put some of the content of the cans into the engine or so. What was it probably? (It was on an cold & icy airport, if that plays a role)

This question may elicit opinion based answers or may be too broad.

Chemicals refill for next chemtrail operation :-(

It was probably Engine Oil.
On modern jet engines, oil is a consumable in the sense that it gets burned up and needs topping up. A new engine will require less, a high time engine more. The JT-9s on older 747 were notorious oil-guzzlers. CF-6s were a bit better but they still needed topping up after an 8 hour flight.
Jet engines never get an oil change. The regular topping-up of burnt oil keeps the oil fresh and regular filter changes ensures material from engine wear is not circulated. Oil samples are taken at set intervals and sent for spectrographic analysis. Too much of a particular element will focus attention on a particular component. There are also 'Magnetic Chip Detectors' which trap ferrous metals and this also helps to pinpoint wear in the engine.


text:
What's the theoretical background of the critical angle of attack?
The critical angle of attack seems to be at all (most?) airfoils around 15-20°. Why is that? Why is it in this range and not lower or higher? Is it just the result of optimizing airfoils? Or is it some inherent property of the air an airfoil is moving through that determines it? (Let's limit the question to subsonic flight.)
PS: I read the answer to "Does airspeed affect the critical angle of attack on an airfoil?" and don't think that this is a duplicate since the other one was more generally about "what are the factors that determine the critical AoA" while I'm asking why the critical AoA seems to be always in this range for most airfoils on planes flying nowadays.

Not an expert, But I imagine that the designer chooses a line somewhere between perfomance v drag, ability to flare and land
too little, inability to fly slow, possibly not be able to climb well, too much, and it flys like a brick

Adding my two cents... You can optimize airfoils for higher angles of attack but this does mean the airfoil becomes garbage for normal flight. As Jeff said, it's a compromise. The reason that we see 15 to 20 degrees probably comes from the regulations, especially max allowed stall speed for your aircraft weight category. If stall speed is dictated and your weight is maxed out for your weight category then - with your chosen flap design and optimal wing area - you will probably need those 15 to 20 degrees of AOA to reach the demanded low stall speed.

Both the airfoil shape and inherent properties of the air contribute to the stall angle of attack. You asked for a theoretical background, but I will list the factors that influence stall because there is no simple formula for it.
The most important factor is the suction peak which develops right behind the stagnation point on the upper side of the leading edge. High suction means high speed and that in turn means high friction, so the air loses energy which it needs further downstream to regain its pressure. If too much energy is lost, the flow separates. Enough separation and lift suffers, so here you have the most immediate reason for a stall.
What can be done to shift that point to a higher angle of attack?

Pitch up faster. That way, the flow over the rear part of the wing has a boundary layer from lower angles of attack and will not separate when the leading edge passes through the angle of attack at wich it stalls in normal conditions. This can shift the stall angle of attack up by 50%. But that works only temporarily and the same mechanism will delay recovery once the stall has occurred.
Increase the leading edge radius. This spreads out the suction peak and makes it less pointy. A blunt leading edge is especially helpful with higher wing loadings when Mach effects play into the stall angle of attack mechanics. Once the local suction peak at the nose of an airfoil reaches a local Mach number of slightly less than 1.6, no lift increase could be observed in experiments.
Increase wing camber, either by nose and/or trailing edge flaps or by cambering the airfoil. This helps to reach high lift coefficients already at low angles of attack, and especially nose devices (slats, Krüger flaps) shift the stall angle of attack up as well.
Use a well-designed airfoil with a long laminar run and a Stratford pressure distribution past the turbulent transition point on the upper side. This helps to reduce boundary layer losses and maximizes energy reserves for the steepest possible pressure rise. But you need a clean, smooth and well built wing for that to really happen. And the right Reynolds number range: Gliders use this effect extensively but airliners cannot use it at all.
Increase wing loading. This will shift the stall speed to a higher Reynolds number where friction losses are smaller relative to the inertial energy of the air. Of course this will increase stall speed, but is will also shift the stall angle of attack higher. A bit.
Increase wing chord (while maintaining the same area). This has actually two effects: The smaller one is again from the increase in Reynolds number, but the more powerful is from the reduction of the wing's aspect ratio. At a smaller aspect ratio the lift curve slope is flatter, so the same lift coefficient (and suction peak) is reached at a higher angle of attack.
Increase wing sweep. The pressure changes over the wing are now proportional to the cosine of the sweep angle, so all effects are shifted to higher angles of attack accordingly. But beware of a combination of high sweep and high aspect ratio: Stall will become outright nasty. If you combine 6 and 7, you will at some point arrive at a delta wing which flies well even with fully separated upper side flow (vortex lift). Now your limit angle of attack will be defined either by the vortex bursting or loss of directional stability.
Fly in hotter, less dense air. This also helps to increase the Reynolds number because you need to move faster for the same dynamic pressure. However, much of that advantage is eaten up by the increase of the air's viscosity with temperature.

While I think I get what you trying to say in Point 1,
Pitch up faster is not something I advice doing when I train in the simulator, as the increased load factor and increase angle of attack will get you into to the stall even quicker. So I am trying to keep it practical on the avoidance of stall, by teaching to reduce the angle of the attack( pitch down) for positive outcome:)

@Herman Of course, that is what you should do. But hysteresis works both ways: On the way up it delays stall and on the way down it delays recovery, too. And that is something to keep in mind when flying quick pitch changes.